It wasnarrated from Anas bin Malik that the Messenger of Allah (ﷺ)said:
"When the Imam says: 'Sami' Allahu liman hamidah (Allah hearsthose who praise Him),' say: 'Rabbana wa lakal-hamd (O our Lord,toYou is the praise).'" (ES)
English reference (ES): Vol. 1, Book 5, Hadith 876 (ES)
Arabic reference (ES): Book 5, Hadith 925 (ES)